Silicon nitride is interesting as a raw material for ceramics materials which are potential substitutes for metals in many fields relating to the construction of apparatus and machinery subjected to heavy wear. These materials are required to be resistant to high temperatures, temperature shocks and corrosion over a wide temperature range.
The Si.sub.3 N.sub.4 powders used for the preparation of such ceramic materials may be obtained by various chemical processes. Among the processes which have already been employed industrially for the production of Si.sub.3 N.sub.4 powder, the process of carbothermal nitridation of SiO.sub.2 (1) and the process of direct nitridation of silicon (2) are important due to the low cost and ready availability of the raw materials. Those processes are shown by the following chemical equations: EQU 3 SiO.sub.2 +6 C+2 N.sub.2 .fwdarw.Si.sub.3 N.sub.4 +6 CO (1) EQU 3 Si+2 N.sub.2 .fwdarw.Si.sub.3 N.sub.4 ( 2)
The thermal and mechanical properties of materials based on Si.sub.3 N.sub.4 depend to a large extent on the nature of the Si.sub.3 N.sub.4 powder used and especially on the metallic and non-metallic impurities present in the powder. Among the non-metallic impurities, both oxygen and the carbon content are of primary importance.
In process (1) which is the carbothermal nitridation of SiO.sub.2 with carbon in an atmosphere containing nitrogen, the reaction product is contaminated with carbon. The amount of carbon contamination is largely determined by the reaction conditions employed.
In the process of direct nitridation of silicon [equation (2)], the carbon contamination is introduced into the Si.sub.3 N.sub.4 powder by impurities present in the raw material, in binders containing carbon or by impurities in the reaction atmosphere such as that present in high temperature nitridation furnaces heated with graphite.
Si.sub.3 N.sub.4 powders contaminated with carbon are inferior in their sintering properties. The sintering densities are adversely influenced by the carbon (H. Hausner, R. Peitzsch in: Keramische Komponenten fur Fahrzeug-Gasturbinen III, Statusseiminar im Auftrag des Bundesministeriums fur Forschung und Technologie, 44-54, Springer-Verlag, Berling, 1984).
Apart from the adverse effect on the sintering characteristics of the Si.sub.3 N.sub.4 powder, contamination with carbon greatly reduces the resistance of Si.sub.3 N.sub.4 materials to oxidation at high temperature so that such materials are unsuitable for use at high temperatures (H. Knoch, G. E. Gazza Journal of the American Ceramic Society 62 (11-12), 634-635, 1979).
Although processes in which Si.sub.3 N.sub.4 powder is produced from very pure compounds such as SiH.sub.4 or SiCl.sub.4 can result in the desired low carbon products, the Si.sub.3 N.sub.4 powder obtained from these processes has the serious disadvantage in that materials obtained from such process by pressing have only low green densities and any subsequent sintering process is accompanied by excessive shrinkage.
A process for the after treatment of Si.sub.3 N.sub.4 powder which has been prepared by direct nitridation of silicon is disclosed in Japanese Patent Application 216,031/83. In this process, the Si.sub.3 N.sub.4 powder is heated in an atmosphere of gaseous chlorine at temperatures above 1100.degree. C., preferably at 1300.degree. C. This heat treatment is followed by annealing in an atmosphere containing nitrogen at temperatures above 1200.degree. C., preferably at 1500.degree. C. These conditions are said to result in a reduction in the carbon content. However, the process has the disadvantage of requiring the use of very high temperatures in corrosive gas atmosphere (such as chlorine) and the furnaces employed must conform to high technical standards.
It was therefore an object of the present invention to provide a process for the after treatment of Si.sub.3 N.sub.4 powder to reduce the carbon content and in which the disadvantages of the processes according to the state of the art would be eliminated.